Re: [PATCH v2 5/5] dax: handle media errors in dax_do_io

Dan Williams
2016-05-02T17:53:29Z

On Mon, May 2, 2016 at 8:18 AM, Jeff Moyer <jmoyer@redhat.com> wrote:
> Dave Chinner <david@fromorbit.com> writes:
[..]
>> We need some form of redundancy and correction in the PMEM stack to
>> prevent single sector errors from taking down services until an
>> administrator can correct the problem. I'm trying to understand
>> where this is supposed to fit into the picture - at this point I
>> really don't think userspace applications are going to be able to do
>> this reliably....
>
> Not all storage is configured into a RAID volume, and in some instances,
> the application is better positioned to recover the data (gluster/ceph,
> for example). It really comes down to whether applications or libraries
> will want to implement redundancy themselves in order to get a bump in
> performance by not going through the kernel. And I think I know what
> your opinion is on that front. :-)
>
> Speaking of which, did you see the numbers Dan shared at LSF on how much
> overhead there is in calling into the kernel for syncing? Dan, can/did
> you publish that spreadsheet somewhere?

Here it is:

https://docs.google.com/spreadsheets/d/1pwr9psy6vtB9DOsc2bUdXevJRz5Guf6laZ4DaZlkhoo/edit?usp=sharing

On the "Filtered" tab I have some of the comparisons where:

noop => don't call msync and don't flush caches in userspace

persist => cache flushing only in userspace and only on individual cache lines

persist_4k => cache flushing only in userspace, but flushing is
performed in 4K aligned units

msync => same granularity flushing as the 'persist' case, but the
kernel internally promotes this to a 4K sized / aligned flush

msync_0 => synthetic case where msync() returns immediately and does
no other work

The takeaway is that msync() is 9-10x slower than userspace cache management.

Let me know if there are any questions and I can add an NVML developer
to this thread...